I came across this DS7 page while helping out Nate, who recently picked up a DS7. There's a good section at the bottom that offers some tips that could be of some use for later Yamaha 2-stroke riders:
Riding technique is important. If you are not in the power band (6000 rpm up), twisting the grip to full throttle is not useful, and actually just helps foul the plugs. Be very patient and wait, or shift gears, until the engine gets to at least 5000 before hitting wide open. Otherwise you are just dumping extra oil into the cylinders and ruining your plugs.
The mufflers are too restrictive even when absolutely clean. They rapidly strangle the machine as they plug up with oily soot. So you need to drill a couple of holes about one eighth or 3/16 of an inch in diameter in the end of each baffle so some exhaust gas can bypass the baffle. It's best to not drill in the chrome area, just beside the exit hole is OK. Then you have to keep the baffles pretty clean, and maybe make a scraping tool to clean them out without always having to remove them. A long screw was what I used. (about 10 inches) I used it kind of like a little rake to scrape out the baffle.
Metallic ash can foul the spark plugs. There are two sources of this, one is leaded gas, which you can't find any more so no worries there. The second source is the 2 stroke oil. Yamaha says use oil "BIA Certified for service TC-W" I don't even need to look that up, 20 years later it's still engraved in my memory. Actually, you sometimes can't find this stuff but what you are looking for is "Ash free" oil. You can check the label. The DS7 ran with lots of oil, and that what partly contributed to fouling but also made the engine last a long time under severe use.
More information and plenty of pictures here.